It is a hypothetical form of energy that permeates space and exerts a negative pressure or “anti-gravity” that repels visible matter and causes the accelerated expansion. On the other hand, dark matter is a form of subatomic particle that does not reflect or emit electromagnetic radiation and causes a positive gravity pressure that attracts visible matter.
Both are inferred rather than observed from observations of gravitational interactions between astronomical objects. As a mathematician I have always been intrigued with the question of why do abstract objects exist and where do they hang out! Mathematics is a good example of abstract ideas. We talk about numbers, sets, propositions, equations, and higher dimensional spaces of more than three dimensions.
Although motivated by the real world, these sorts of ideas and objects are not part of the physical world as we know it. Given certain assumptions and these abstract objects, we can then use mathematics to derive certain formulae about the objects. Our abstract idea of a triangle is irrespective of its shape, size, and color, and although it has many physical representations its “essence” or “form” as Plato would describe it, does not exist in the material world.